How is overflow detected at the binary level?

Question

I'm reading the textbook Computer Organization And Design by Hennessey and Patterson (4th edition). On page 225 they describe how overflow is detected in signed, 2's complement arithmetic. I just can't even understand what they're talking about.

"How do we detect [overflow] when it does occur? Clearly, adding or substracting two 32-bit numbers can yield a result that needs 33 bits to be fully expressed."

Sure. And it won't need 34 bits because even the smallest 34 bit number is twice the smallest 33 bit number, and we're adding 32 bit numbers.

"The lack of a 33rd bit means that when overflow occurs, the sign bit is set with the value of the result instead of the proper sign of the result."

What does this mean? The sign bit is set with the "value" of the result... meaning it's set as if the result were unsigned? And if so, how does that follow from the lack of a 33rd bit?

"Since we need just one extra bit, only the sign bit can be wrong."

And that's where they lost me completely.

What I'm getting from this is that, when adding signed numbers, there's an overflow if and only if the sign bit is wrong. So if you add two positives and get a negative, or if you add two negatives and get a positive. But I don't understand their explanation.

Also, this only applies to unsigned numbers, right? If you're adding signed numbers, surely detecting overflow is much simpler. If the last half-adder of the ALU sets its carry bit, there's an overflow.

note: I really don't know what tags are appropriate here, feel free to edit them.

Answer 1

Any time you want to deal with these kind of ALU items be it add, subtract, multiply, etc, start with 2 or 3 bit numbers, much easier to get a handle on than 32 or 64 bit numbers. After 2 or 3 bits it doesn't matter if it is 22 or 2200 bits it all works exactly the same from there on out. Basically you can by hand if you want make a table of all 3 bit operands and their results such that you can examine the whole table visually, but a table of all 32 bit operands against all 32 bit operands and their results, can't do that by hand in a reasonable time and cannot examine the whole table visually.

Now twos complement, that is just a scheme for representing positive and negative numbers, and it is not some arbitrary thing it has a reason, the reason for the madness is that your adder logic (which is also what the subtractor uses which is the same kind of thing the multiplier uses) DOES NOT CARE ABOUT UNSIGNED OR SIGNED. It does not know the difference. YOU the programmer cares in my three bit world the bit pattern 0b111 could be a positive seven (+7) or it could be a negative one. Same bit pattern, feed it to the add logic and the same thing comes out, and the answer that comes out I can choose to interpret as unsigned or twos complement (so long as I interpret the operands and the result all as either unsigned or all as twos complement). Twos complement also has the feature that for negative numbers the most significant bit (msbit) is set, for positive numbers it is zero. So it is not sign plus magnitude but we still talk about the msbit being the sign bit, because except for two special numbers that is what it is telling us, the sign of the number, the other bits are actually telling us the magnitude they are just not an unsigned magnitude as you might have in sign+magnitude notation.

So, the key to this whole question is understanding your limits. For a 3 bit unsigned number our range is 0 to 7, 0b000 to 0b111. for a 3 bit signed (twos complement) interpretation our range is -4 to +3 (0b100 to 0b011). For now limiting ourselves to 3 bits if you add 7+1, 0b111 + 0b001 = 0b1000 but we only have a 3 bit system so that is 0b000, 7+1 = 8, we cannot represent 8 in our system so that is an overflow, because we happen to be interpreting the bits as unsigned we look at the "unsigned overflow" which is also known as the carry bit or flag. Now if we take those same bits but interpret them as signed, then 0b111 (-1) + 0b001 (+1) = 0b000 (0). Minus one plus one is zero. No overflow, the "signed overflow" is not set...What is the signed overflow?

First what is the "unsigned overflow".

The reason why "it all works the same" no matter how many bits we have in our registers is no different than elementary school math with base 10 (decimal) numbers. If you add 9 + 1 which are both in the ones column you say 9 + 1 = zero carry the 1. you carry a one over to the tens column then 1 plus 0 plus 0 (you filled in two zeros in the tens column) is 1 carry the zero. You have a 1 in the tens column and a zero in the ones column:

  1
  09
 +01
====
  10

What if we declared that we were limited to only numbers in the ones column, there isn't any room for a tens column. Well that carry bit being a non-zero means we have an overflow, to properly compute the result we need another column, same with binary:

  111 
   111
 + 001
=======
  1000

7 + 1 = 8, but we cant do 8 if we declare a 3 bit system, we can do 7 + 1 = 0 with the carry bit set. Here is where the beauty of twos complement comes in:

  111 
   111
 + 001
=======
   000

if you look at the above three bit addition, you cannot tell by looking if that is 7 + 1 = 0 with the carry bit set or if that is -1 + 1 = 0.

So for unsigned addition, as we have known since grade school that a carry over into the next column of something other than zero means we have overflowed that many placeholders and need one more placeholder, one more column, to hold the actual answer.

Signed overflow. The sort of academic answer is if the carry in of the msbit column does not match the carry out. Let's take some examples in our 3 bit world. So with twos complement we are limited to -4 to +3. So if we add -2 + -3 = -5 that wont work correct? To figure out what minus two is we do an invert and add one 0b010, inverted 0b101, add one 0b110. Minus three is 0b011 -> 0b100 -> 0b101

So now we can do this:

  abc
  100 
   110
 + 100
======
   010

If you look at the number under the b that is the "carry in" to the msbit column, the number under the a the 1, is the carry out, these two do not match so we know there is a "signed overflow".

Let's try 2 + 2 = 4:

  abc
  010
   010
 + 010
======
   100

You may say but that looks right, sure unsigned it does, but we are doing signed math here, so the result is actually a -4 not a positive 4. 2 + 2 != -4. The carry in which is under the b is a 1, the carry out of the msbit is a zero, the carry in and the carry out don't match. Signed overflow.

There is a shortcut to figuring out the signed overflow without having to look at the carry in (or carry out). if ( msbit(opa) == msbit(opb) ) && ( msbit(res) != msbit(opb) ) signed overflow, else no signed overflow. opa being one operand, opb being the other and res the result.

   010
 + 010
======
   100

Take this +2 + +2 = -4. msbit(opa) and msbit(opb) are equal, and the result msbit is not equal to opb msbit so this is a signed overflow. You could think about it using this table:

x ab cr 
0 00 00
0 01 01
0 10 01
0 11 10 signed overflow
1 00 01 signed overflow
1 01 10
1 10 10
1 11 11

This table is all the possible combinations if carry in bit, operand a, operand b, carry out and result bit for a single column turn your head sideways to the left to sort of see this x is the carry in, a and b columns are the two operands. cr as a pair is the result xab of 011 means 0+1+1 = 2 decimal which is 0b10 binary. So taking the rule that has been dictated to us, that if the carry in and carry out do not match that is a signed overflow. Well the two cases where the item in the x column does not match the item in the c column are indicated those are the cases where a and b inputs match each other, but the result bit is the opposite of a and b. So assuming the rule is correct this quick shortcut that does not require knowing what the carry bits are, will tell you if there was a signed overflow.

Now you are reading an H&P book. Which probably means mips or dlx, neither mips or dlx deal with carry and signed flags in the way that most other processors do. mips is not the best first instruction set IMO primarily for that reason, their approach is not wrong in any way, but being the oddball, you will spend forever thinking differently and having to translate when going to most other processors. Where if you learned the typical znvc flags (zero flag, negative flag, v=signed overflow, c=carry or unsigned overflow) way then you only have to translate when going to mips. Normally these are computed on every alu operation (for the non-mips type processors) you will see signed and unsigned overflow being computed for add and subtract. (I am used to an older mips, maybe this gen of books and the current instruction set has something different). Calling it addu add unsigned right at the start of mips after learning all of the above about how an adder circuit does not care about signed vs unsigned, is a huge problem with mips it really puts you in the wrong mindset for understanding something this simple. Leads to the belief that there is a difference between signed addition and unsigned addition when there isn't. It is only the overflow flags that are computed differently. Now multiply, and divide there is definitely a twos complement vs unsigned difference and you ideally need a signed multiply and an unsigned multiply or you need to deal with the limitation.

I recommend a simple (depending on how strong your bit manipulation is and twos complement) exercise that you can write in some high level language. Basically take all the combinations of unsigned numbers 0 to 7 added to 0 to 7 and save the result. Print out both as decimal and as binary (three bits for operands, four bits for result) and if the result is greater than 7 print overflow as well. Repeat this using signed variables using the numbers -4 to +3 added to -4 to +3. print both decimal with a +/- sign and the binary. If the result is less than -4 or greater than +3 print overflow. From those two tables you should be able to see that the rules above are true. Looking strictly at the operand and result bit patterns for the size allowed (three bits in this case) you will see that the addition operation gives the same result, same bit pattern for a given pair of inputs independent of whether those bit patterns are considered unsigned or twos complement. Also you can verify that unsigned overflow is when the result needs to use that fourth column, there is a carry out off of the msbit. For signed when the carry in doesn't match the carry out, which you see using the shortcut looking at the msbits of the operands and result. Even better is to have your program do those comparisons and print out something. So if you see a note in your table that the result is greater than 7 and a note in your table that bit 3 is set in the result, then you will see for the unsigned table that is always the case (limited to inputs of 0 to 7). And the more complicated one, signed overflow, is always when the result is less than -4 and greater than 3 and when the operand upper bits match and the result upper bit does not match the operands.

I know this is super long and very elementary. If I totally missed the mark here, please comment and I will remove or re-write this answer.

The other half of the twos complement magic. Hardware does not have subtract logic. One way to "convert" to twos complement is to "invert and add one". If I wanted to subtract 3 - 2 using twos complement what actually happens is that is the same as +3 + (-2) right, and to get from +2 to to -2 we invert and add one. Looking at our elementary school addition, did you notice the hole in the carry in on the first column?

  111H
   111
 + 001
=======
  1000

I put an H above where the hole is. Well that carry in bit is added to the operands right? Our addition logic is not a two input adder it is a three input adder yes? Most of the columns have to add three one bit numbers in order to compute two operands. If we use a three input adder on the first column now we have a place to ... add one. If I wanted to subtract 3 - 2 = 3 + (-2) = 3 + (~2) + 1 which is:

    1
  011
+ 101
=====

Before we start and filled in it is:

 1111
  011
+ 101
=====
  001

3 - 2 = 1.

What the logic does is:

if add then carry in = 0; the b operand is not inverted, the carry out is not inverted. if subtract then carry in = 1; the b operand is inverted, the carry out MIGHT BE inverted.

The addition above shows a carry out, I didn't mention that this was an unsigned operation 3 - 2 = 1. I used some twos complement tricks to perform an unsigned operation, because here again no matter whether I interpret the operands as signed or unsigned the same rules apply for if add or if subtract. Why I said that the carry out MIGHT BE inverted is that some processors invert the carry out and some don't. It has to do with cascading operations, taking say a 32 bit addition logic and using the carry flag and an add with carry or subtract with borrow instruction creating a 64 bit add or subtract, or any multiple of the base register size. Say you have two 64 bit numbers in a 32 bit system a:b + c:d where a:b is the 64 bit number but it is held in the two registers a and b where a is the upper half and b is the lower half. so a:b + c:d = e:f on a 32 bit system unsigned that has a carry bit and add with carry:

add f,b,d
addc e,a,c

The add leaves its carry out bit from the upper most bit lane in the carry flag in the status register, the addc instruction is add with carry takes the operands a+c and if the carry bit is set then adds one more. a+c+1 putting the result in e and the carry out in the carry flag, so:

add f,b,d
addc e,a,c
addc x,y,z

Is a 96 bit addition, and so on. Here again something very foreign to mips since it doesn't use flags like other processors. Where the invert or don't invert comes in for signed carry out is on the subtract with borrow for a particular processor. For subtract:

if subtract then carry in = 1; the b operand is inverted, the carry out MIGHT BE inverted.

For subtract with borrow you have to say if the carry flag from the status register indicates a borrow then the carry in is a 0 else the carry in is a 1, and you have to get the carry out into the status register to indicate the borrow.

Basically for the normal subtract some processors invert b operand and carry on in the way in and carry out on the way out, some processors invert the b operand and carry in in the way in but don't invert carry out on the way out. Then when you want to do a conditional branch you need to know if the carry flag means greater than or less than (often the syntax will have a branch if greater or branch if less than and sometimes tell you which one is the simplified branch if carry set or branch if carry clear). (If you don't "get" what I just said there that is another equally long answer which won't mean anything so long as you are studying mips).